1. Field of the Invention
The present invention relates to an image processing apparatus, a method for controlling the same, and a storage medium.
2. Description of the Related Art
When transmission of a fax document coincides with the reception of a phone call from a third party (coinciding of an outgoing call and an incoming call), a line can be erroneously connected in some cases. If the third party has a function of switching to a fax reception mode upon receiving a FAX calling signal (CNG), the fax document may end up being sent to the third party. To prevent such an erroneous transmission, there is a method in which a dial tone incoming from a switch is monitored at the time of calling and unless the dial tone is detected, the calling (transmission) is not carried out.
Standards for dial tones vary between countries. Frequencies and cadence patterns, which define the On-time and Off-time of the dial tone, vary from country to country. In order to prevent erroneous transmission of a fax document as described above, it is necessary to correctly detect dial tones having differing patterns in different countries. Furthermore, when an outgoing call and an incoming call collide, a sound may be audible from the line, so it is also very important not to misdetect such a sound as the dial tone.
To make a cadence check of a tone signal, Japanese Patent Laid-Open No. 10-210113 proposes preparing a pattern table that specifies an upper limit value and a lower limit value for each of the On-time and the Off-time country by country. Based on the upper limit and lower limit values in the table, it is determined whether the On-time and Off-time of a received tone signal are within the respective ranges.
However, the conventional technology has the problems given below. Generally speaking, from the viewpoint of program maintenance, it is preferable that the pattern table prepared is small. In the method of the conventional technology described above, to have standards of a plurality of countries covered with one pattern table, the upper limit and lower limit values of the ON-time and OFF-time need to be expanded. With the expanded upper limit and lower limit values of ON-time and OFF-time, it is possible to detect any cadence of the received dial tone to some extent.
On the other hand, that increases the possibility of misdetection when the sound is received. Some of the sounds include the same frequency as that of the dial tone, and thus receiving the sound causes a filter for detecting the dial tone intermittently to be turned ON/OFF. With the expanded upper limit and lower limit values of On-time and Off-time, there is an increased possibility that the ON/OFF happens to be within the valid range even when the sound is received and as a result, a misdetection occurs. In that case, even though there is no dial tone, the misdetection allows the call to go out, leading to erroneous transmission of a fax document.